This invention relates to a method and apparatus for monitoring a redundant (or standby) transmitter in a radio communications system having an active transmitter and one or more receivers. The invention is especially, although not exclusively, concerned with point-to-multipoint radio communications systems.
Radio transmission systems are often used to transmit data. A common scenario is the transmission of data from a base station to a number of receivers in a point-to-multipoint system. Such an arrangement is shown in FIG. 1, in which the base station (BS) communicates with a number of terminal stations (TS1-TS3) in one particular sector 10 of the base station's range. To reduce the impact of equipment failure (i.e., to increase the availability of a radio link) it is known practice to install duplicate equipment (standby equipment) that is redundant whilst the equipment functions correctly. In the case of a point-to-multipoint system, since the integrity of the base-station transmitter facility is of prime importance, it is that transmitter that is normally duplicated. This is illustrated in FIG. 2, in which the active and passive (redundant or standby) transmitters are basically identical and comprise an indoor part (IDU) and a cable-linked outdoor part (ODU). The ODU includes an RF stage and an antenna 11. Where failure of the active transmitter occurs, the redundant (standby) transmitter can take over the transmission of data.
There is the possibility, however, that the redundant transmitter will fail before the active transmitter fails. This is especially likely when the redundant transmitter is permanently energised (so-called “hot standby”). Failure then has a probability of 50%. If failure of the redundant transmitter is not detected, the redundancy is lost and, in the event that the active equipment also fails, data transmission is completely lost. This is obviously particularly disastrous in point-to-multipoint systems, since a whole sector can be lost with the failure of the base station.
There is, therefore, a need to monitor the integrity of the redundant transmitter in such a communications system.
The supervision of a redundant transmitter, however, poses an acute problem, as a transmitter can only be fully tested by transmitting a signal, which in turn may adversely affect the transmission of data.
At the present time, three methods of supervision are known:                the data signal uses both transmitters (active and redundant) at different frequencies or time slots. This is possible only in point-to-multipoint systems if the data signal in the downlink is “bursty”, i.e. if different time slots are employed for different terminal stations, or if it is an FDM (Frequency-Division Multiplex) signal, i.e. different frequency bands are used for different terminal stations.        a pilot signal is transmitted over the redundant transmitter, either in the time or frequency domain. This is, however, a waste of resources.        the outdoor unit (ODU) is self-monitoring, i.e. the supervision is performed only at the analogue part of the transmitter, for example by monitoring the oscillators. This requires a special development of the redundant ODU and a digital link between the redundant ODU and IDU (indoor unit). Further, this involves additional hardware outlay, which can be costly.        
Since in the case of a TDMA (Time-Division Multiple Access) point-to-multipoint system the downlink signal is time-continuous, the first method for supervision cannot be used. The other two have the drawback that they are inefficient.